This invention relates to electrochromic displays which produce a chemical color change when exposed to an electromagnetic stimulus, and, more particularly to large area, thin film electrochromic displays.
In a typical electrochromic display, a variable and reversible potential or current is applied across outer electrodes to produce reversible color changes in the electrochromic material. In thin film displays, a thin film electrochromic material is positioned against a thin film electrolyte and both films are sandwiched between two electrodes.
Such devices can be used in indoor or outdoor displays, electronic data displays, advertising and clocks. A potential use is in adjusting the amount of solor energy passing through windows, thus reducing the heating, cooling and lighting expenses of buildings.
Since electrochromic devices should exhibit ionic as well as electronic conduction with associated reversible and visible optical absorption, they generally use liquid electrolytes. Such electrolytes are highly acidic or corrosive salt solutions enclosed in leaktight and inert containers. Chemical degradation and electrode deterioration reduce their useful life times. These devices have failed to reach commercialization largely because of chemical and physical problems associated with liquid electrolyes.
As a result, an attempt has been made to employ a solid-state electrolyte. One such device is based on metal oxides. This U.S. Pat. No. 3,521,941, where magnesium fluoride (MgFl.sub.2) is used to supply protons and several insulator layers are used to stop electron flow. Unfortunately, these devices are irreproducible and suffer from imperfect reversibility.
Commercially viable devices have yet to be produced because of the uncontrolability of the chemical species that have been used to date.
Accordingly, it is an object of the invention to facilitate the production of electrochromic materials. A related object is to overcome the shortcomings of the prior art. Another object is to do so for large-area panels. A further related object is to provide improved color contrast and wide viewing angles.
Still another object of the invention is to increase the cycle life of electrochromic devices and decrease the response time at relatively low cost.
A further object of the invention is to achieve low power consumption for devices with fast recovery from overload and short circuit conditions.
Yet another object of the invention is to achieve photochromic devices that are easily patterned.
Still another object of the invention is to achieve relative freedom from maintenance, with improved reliability at variable operating temperatures and simplified fabrication. A related object is to realize electrochromic devices with flexible substrates that can assume any prescribed size or shape without extra encapsulation cost.